Should I be getting better gas mileage? Improvements?

[cheetahranger]

Okay, so I've been battling bad gas mileage ever since I got my truck a little over a year ago from my dad. He also got it from his, so I know everything ever done to it for the most part. It's a 1993 SCSB GMC Sierra 1500 with a 5.7l and the 4l60e. Its got ~241k miles on the body, 130k on the 5.7l which was swapped in for better towing power than it's original 5.0l. It also has a newer 4l60e, with approximately 30k miles on it. Side note: it will shift hard from 1-2 unless floored. I've checked my TPS sensor voltage and it seems fine. Before the original transmission was replaced, it had a shift kit, so maybe during the replacement the PCM or line pressure stuff wasn't adjusted back? Any help on that would be appreciated, but it never honestly affects my driving and it's not banging into gear. Also lurches when put in park and reverse. So, for the mileage... I get about 9.5 mpg city adjusted 10.5% for the difference in revolution/mile with tires. I have 285/70/17 (~33in) Yokohama Geolandar MT03s on GFX-TR19 rims with 3.42 gears. Aired up to ~30 psi all around. No lift, just cranked to level. My exhaust setup is the stock one for the 5.7l, with no muffler into a non-mandrel Y-pipe side exit, with a stock (I'm guessing replaced w/ the engine) cat. In search of better MPGs, I've switched from a 165 degree thermostat to a 180. There are no coolant leaks whatsoever. I've also replaced the engine coolant temperature sensor, O2 sensor, all engine and driveline fluids with synthetic, spark plugs (which honestly I forgot to gap) and wires, distributor cap and rotor, PCV valve, ignition coil, air filter, alternator, fan clutch, fuel filter, brake light switch, oil filters. I've done the IAC valve after getting a code for it, which still turns on when idling, and then turns off when driving. I did the relearn procedure, and it still persists, however doesn't affect performance at all. Idles ~1100 in park and neutral, ~700 in gear. Anybody know why? My EGR vacuum line from the solenoid to valve also broke and I've yet to replace it, but would it make a big difference at non-highway speeds? The TBI system isn't the cleanest, I should probably spray it down with throttle body cleaner. The injectors have a great cone pattern no drips. Back to mileage, my truck weighs ~5200lbs with a loaded toolbox, headache rack, custom off-road bumper I built plus a 12k winch. The truck runs like a top so I'm not sure why it's so low. I don't expect great mileage out of her, but 9.5 seems poor. I try not to drive too aggressively, but I occasionally have fun . Any knowledge would be appreciated!

 

[Schurkey]

Install a proper thermostat. Lose the bigass tires and any excess weight.

Connect a scan tool, find out what the computer is trying to tell you.
Fuel trims, knock sensor, spark advance--verify all the sensors and computer outputs. There's a reason the idle is way too high, and that's why it jerks when you put it in gear.

No functional EGR probably means the knock sensor is going nuts, and you have mega-retarded timing. Fix the EGR.

Make sure the brakes don't drag

 

[cheetahranger]

Install a proper thermostat. Lose the bigass tires and any excess weight.

Connect a scan tool, find out what the computer is trying to tell you.
Fuel trims, knock sensor, spark advance--verify all the sensors and computer outputs. There's a reason the idle is way too high, and that's why it jerks when you put it in gear.

No functional EGR probably means the knock sensor is going nuts, and you have mega-retarded timing. Fix the EGR.

Make sure the brakes don't drag

By proper thermostat do you mean a 195? It's OBD1, so I'd need to buy one of those other tools that connect a usb to a laptop? Good to know the EGR is that important, thought it just was a stupid emissions thing didn't realize it could effect timing. Thanks!

 

[99Yak]

He's correct. Go back to 245's.
I lost 2 mpg moving up a size. (K1500 Burb)

 

[Supercharged111]

I can't see the tires tanking fuel economy that badly. I run the same size but with more gear and an assload more torque. Something else is wrong here. 3.42 and a 33 is a lot, but not the cause for single digit fuel economy. That's worse than my supercharged big block dually gets.

 

[cheetahranger]

He's correct. Go back to 245's.
I lost 2 mpg moving up a size. (K1500 Burb)

Yea the only issue is I'm building it into a trail rig, gonna be putting a lift and locker and regear to 4.10s, so while none of that is good for fuel mileage I'm trying to get the best I can.

 

[L31MaxExpress]

350 should have a 350 Memcal, injectors and knock sensor to run at its peak. My 83 G20 with a stock long block HD TBI 350, 4L60E and 3.08s would get better than 20 mpg highway and 13-14 around town. Mine had 1.6 roller rockers, tri y headers, factory 2.5" catless duals that I added a X-pipe to, an edelbrock 3704 intake bored to 2" and a marine 2" TBI unit with chip tuning.

 

[letitsnow]

"In Town" mpg.

 

[Pinger]

Aired up to ~30 psi all around.

A bit low - no?
I'm running 45psi at the front, 50psi at the rear. 265/65 17 on a C2500 Suburban.

 

[TreeGeared]

The wider tires could be contributing. You are basically moving a given mass of a given weight through a body of air that is resisting your motion. By lifting or using larger tires you are increasing your front cross sectional area. Airflow under a vehicle is terribly inefficient. The weight you have added is not helping either.

Your gears are more for highway but they may be hurting you with the larger tires because it keeps your engine out of a good spot in the torque curve.

Lastly did you update the memcal for the larger displacement engine and possibly different cam? These computers are not very adaptive to changing components.